Replace dark mode with player-adjustable lighting (#4280)

## What

Removes the binary **dark mode** feature and replaces it with a
player-adjustable **Lighting** section in graphics settings.

### In-game settings
- Removed the Dark Mode toggle from both `SettingsModal` and
`UserSettingModal`, and `darkMode()`/`toggleDarkMode()`/`DARK_MODE_KEY`
from `UserSettings`.

### New Lighting section (Graphics Settings)
- **Ambient light** slider (1–3): mapped to the renderer's ambient as
`ambient = 1 / level`. **1.0 = no effect (unchanged look), 3.0 = darkest
with the strongest structure glow.**
- **Light falloff** slider (1–3): writes straight to
`lighting.falloffPower`.
- Lighting auto-enables only when ambient < 1, so the default (slider at
1) has zero GPU cost — off by default.

### Removed dark-mode overrides
- Deleted `applyDarkModeOverride()` + `DARK_AMBIENT` and their wiring in
`ClientGameRunner`, `gl/index.ts`, and the `DARK_MODE_KEY` listener.
- Removed the `.dark` HUD-class toggle in `Main.ts` and the
`userSettings.darkMode()` read in `PlayerIcons`.

### Train glow
- `UT_TRAIN` light reduced (intensity `2.0 → 0.5`, radius `8 → 6`) so
structures dominate the glow.

## Notes
- Removing the dark-mode setting also retires the HUD's Tailwind dark
theme (same setting). The dormant `dark:` CSS variants and unused
white-icon assets are left in place (out of scope).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/core/game/UserSettings.ts

@@ -54,7 +54,6 @@ export const USER_SETTINGS_CHANGED_EVENT = "event:user-settings-changed";
 export const PATTERN_KEY = "territoryPattern";
 export const FLAG_KEY = "flag";
 export const COLOR_KEY = "settings.territoryColor";
-export const DARK_MODE_KEY = "settings.darkMode";
 export const PERFORMANCE_OVERLAY_KEY = "settings.performanceOverlay";
 export const KEYBINDS_KEY = "settings.keybinds";
 export const GRAPHICS_KEY = "settings.graphics";
@@ -154,10 +153,6 @@ export class UserSettings {
     return this.getBool("settings.lobbyIdVisibility", true);
   }
 
-  darkMode() {
-    return this.getBool(DARK_MODE_KEY, false);
-  }
-
   leftClickOpensMenu() {
     return this.getBool("settings.leftClickOpensMenu", false);
   }
@@ -235,10 +230,6 @@ export class UserSettings {
     this.setBool("settings.goToPlayer", !this.goToPlayer());
   }
 
-  toggleDarkMode() {
-    this.setBool(DARK_MODE_KEY, !this.darkMode());
-  }
-
   // For development only. Used for testing patterns, set in the console manually.
   getDevOnlyPattern(): PlayerPattern | undefined {
     const data = localStorage.getItem("dev-pattern") ?? undefined;